Client-server computing models include a distributed application structure that divides tasks between service providers (servers) and service requesters (clients). Clients and servers communicate over computer networks where clients request content on the servers or service functions provided by the servers. Clients initiate communication sessions with servers, which monitor incoming requests. Servers share their content or functions or computing resources with the requesting clients. Clients and servers form two tiers of a computer architecture, where the first tier, the servers, provides information based upon queries submitted by the second tier, the client application.
Servers share functions with clients such as email exchange, web access, and database access. A web browser is a client program running on a client's computer that accesses information stored on a web server on the Internet. Other users access banking services from their computers with a web browser client that sends a request to a web server at a bank. The bank program might perform additional functions on the client request such as forwarding the request to a separate database client program that sends a request to a database server at another bank computer to retrieve account information, such as an account balance. The balance information is then returned to the bank database client, which in turn serves it back to the web browser client displaying the results to the user.
The many client requests and server responses provide many points on the network where communication may be interrupted or service of the requests can fail. To reduce the frequency of communication interruptions and service request failures, a distributed service may employ data replication, where a service maintains multiple copies of data to permit local access at multiple locations, or to increase availability when a server process may have crashed. Data caching techniques, where a copy of requested data is copied and retained locally, can also be used to reduce the number and frequency of service calls made from a client to a server. For example, web browsers use document caching to speed up access to frequently used documents. Data caching is similar to data replication, but cached data can become stale when it remains on a local computer too long. Other systems employ other techniques to address communication interruptions and service failures.
Resilient computer systems are those computer systems that tolerate faults or communication interruptions or service request failures and provide an acceptable quality of service in spite of the interruptions. Resilient computer systems can withstand service interruptions without permanently failing. Resilient systems recover from service interruptions and make adjustments to continue to provide service.
In order to recover quickly or provide service alternatives, a resilient computer system or computer application needs to recognize the temporary loss of communication connection between the client and the server. The client application must be aware that the connection will be temporarily unavailable and must retry access rather than ending or initiating a failover. Similarly, server applications need to be aware that the communication connection is no longer available. Different approaches have been used in the past to provide resilient computer systems that provide persistent service delivery and can justifiably be trusted when servicing user requests.